1 / 83

Instructors: Dave O’Hallaron, Greg Ganger, and Greg Kesden

Bits, Bytes, and Integers 15-213: Introduction to Computer Systems 2 nd and 3 rd Lectures, Sep 1 and Sep 6, 2011. Instructors: Dave O’Hallaron, Greg Ganger, and Greg Kesden. Today: Bits, Bytes, and Integers. Representing information as bits Bit-level manipulations Integers

sydnee
Télécharger la présentation

Instructors: Dave O’Hallaron, Greg Ganger, and Greg Kesden

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Bits, Bytes, and Integers15-213: Introduction to Computer Systems2nd and 3rd Lectures, Sep 1 and Sep 6, 2011 Instructors: Dave O’Hallaron, Greg Ganger, and Greg Kesden

  2. Today: Bits, Bytes, and Integers • Representing information as bits • Bit-level manipulations • Integers • Representation: unsigned and signed • Conversion, casting • Expanding, truncating • Addition, negation, multiplication, shifting • Summary • Representations in memory, pointers, strings

  3. 0 1 0 3.3V 2.8V 0.5V 0.0V Binary Representations • Base 2 Number Representation • Represent 1521310 as 111011011011012 • Represent 1.2010 as 1.0011001100110011[0011]…2 • Represent 1.5213 X 104 as 1.11011011011012 X 213 • Electronic Implementation • Easy to store with bistable elements • Reliably transmitted on noisy and inaccurate wires

  4. 15 14 10 13 11 12 0011 1111 1110 0000 0001 0010 1100 1101 1011 0100 0101 1010 0110 1001 0111 1000 3 8 8 4 5 7 6 6 F 4 2 5 2 A 1 9 D B 1 9 E 7 3 0 0 C Decimal Binary Hex Encoding Byte Values • Byte = 8 bits • Binary 000000002 to 111111112 • Decimal: 010 to 25510 • Hexadecimal 0016 to FF16 • Base 16 number representation • Use characters ‘0’ to ‘9’ and ‘A’ to ‘F’ • Write FA1D37B16 in C as • 0xFA1D37B • 0xfa1d37b

  5. Data Representations

  6. Today: Bits, Bytes, and Integers • Representing information as bits • Bit-level manipulations • Integers • Representation: unsigned and signed • Conversion, casting • Expanding, truncating • Addition, negation, multiplication, shifting • Summary • Representations in memory, pointers, strings

  7. Boolean Algebra • Developed by George Boole in 19th Century • Algebraic representation of logic • Encode “True” as 1 and “False” as 0 • And • A&B = 1 when both A=1 and B=1 • Or • A|B = 1 when either A=1 or B=1 • Not • ~A = 1 when A=0 • Exclusive-Or (Xor) • A^B = 1 when either A=1 or B=1, but not both

  8. General Boolean Algebras • Operate on Bit Vectors • Operations applied bitwise • All of the Properties of Boolean Algebra Apply 01101001 & 01010101 01000001 01101001 | 01010101 01111101 01101001 ^ 01010101 00111100 ~ 01010101 10101010 01000001 01111101 00111100 10101010

  9. Example: Representing & Manipulating Sets • Representation • Width w bit vector represents subsets of {0, …, w–1} • aj = 1 if j ∈ A • 01101001 { 0, 3, 5, 6 } • 76543210 • 01010101 { 0, 2, 4, 6 } • 76543210 • Operations • & Intersection 01000001 { 0, 6 } • | Union 01111101 { 0, 2, 3, 4, 5, 6 } • ^ Symmetric difference 00111100 { 2, 3, 4, 5 } • ~ Complement 10101010 { 1, 3, 5, 7 }

  10. Bit-Level Operations in C • Operations &, |, ~, ^ Available in C • Apply to any “integral” data type • long, int, short, char, unsigned • View arguments as bit vectors • Arguments applied bit-wise • Examples (Char data type) • ~0x41 ➙ 0xBE • ~010000012 ➙ 101111102 • ~0x00 ➙ 0xFF • ~000000002 ➙ 111111112 • 0x69 & 0x55 ➙ 0x41 • 011010012 & 010101012 ➙ 010000012 • 0x69 | 0x55 ➙ 0x7D • 011010012 | 010101012➙ 011111012

  11. Contrast: Logic Operations in C • Contrast to Logical Operators • &&, ||, ! • View 0 as “False” • Anything nonzero as “True” • Always return 0 or 1 • Early termination • Examples (char data type) • !0x41 ➙ 0x00 • !0x00 ➙ 0x01 • !!0x41 ➙ 0x01 • 0x69 && 0x55 ➙ 0x01 • 0x69 || 0x55 ➙ 0x01 • p && *p (avoids null pointer access)

  12. Contrast: Logic Operations in C • Contrast to Logical Operators • &&, ||, ! • View 0 as “False” • Anything nonzero as “True” • Always return 0 or 1 • Early termination • Examples (char data type) • !0x41 ➙ 0x00 • !0x00 ➙ 0x01 • !!0x41 ➙ 0x01 • 0x69 && 0x55 ➙ 0x01 • 0x69 || 0x55 ➙ 0x01 • p && *p (avoids null pointer access) Watch out for && vs. & (and || vs. |)… one of the more common oopsies in C programming

  13. Argument x Argument x Arith. >> 2 Arith. >> 2 Log. >> 2 Log. >> 2 11101000 01100010 00010000 00011000 00011000 10100010 00010000 00101000 11101000 00101000 00010000 11101000 00101000 00010000 00011000 00011000 00011000 00010000 00010000 00011000 << 3 << 3 Shift Operations • Left Shift: x << y • Shift bit-vector x left y positions • Throw away extra bits on left • Fill with 0’s on right • Right Shift: x >> y • Shift bit-vector x right y positions • Throw away extra bits on right • Logical shift • Fill with 0’s on left • Arithmetic shift • Replicate most significant bit onleft • Undefined Behavior • Shift amount < 0 or ≥ word size

  14. Today: Bits, Bytes, and Integers • Representing information as bits • Bit-level manipulations • Integers • Representation: unsigned and signed • Conversion, casting • Expanding, truncating • Addition, negation, multiplication, shifting • Summary • Representations in memory, pointers, strings • Summary

  15. Encoding Integers Unsigned Two’s Complement • C short 2 bytes long • Sign Bit • For 2’s complement, most significant bit indicates sign • 0 for nonnegative • 1 for negative short int x = 15213; short int y = -15213; Sign Bit

  16. Encoding Example (Cont.) x = 15213: 00111011 01101101 y = -15213: 11000100 10010011

  17. Unsigned Values UMin = 0 000…0 UMax = 2w – 1 111…1 Two’s Complement Values TMin = –2w–1 100…0 TMax = 2w–1 – 1 011…1 Other Values Minus 1 111…1 Numeric Ranges Values for W = 16

  18. Values for Different Word Sizes • Observations • |TMin| = TMax + 1 • Asymmetric range • UMax = 2 * TMax + 1 • C Programming • #include<limits.h> • Declares constants, e.g., • ULONG_MAX • LONG_MAX • LONG_MIN • Values platform specific

  19. X B2U(X) B2T(X) 0000 0 0 0001 1 1 0010 2 2 0011 3 3 0100 4 4 0101 5 5 0110 6 6 0111 7 7 1000 8 –8 1001 9 –7 1010 10 –6 1011 11 –5 1100 12 –4 1101 13 –3 1110 14 –2 1111 15 –1 Unsigned & Signed Numeric Values • Equivalence • Same encodings for nonnegative values • Uniqueness • Every bit pattern represents unique integer value • Each representable integer has unique bit encoding •  Can Invert Mappings • U2B(x) = B2U-1(x) • Bit pattern for unsigned integer • T2B(x) = B2T-1(x) • Bit pattern for two’s comp integer

  20. Today: Bits, Bytes, and Integers • Representing information as bits • Bit-level manipulations • Integers • Representation: unsigned and signed • Conversion, casting • Expanding, truncating • Addition, negation, multiplication, shifting • Summary • Representations in memory, pointers, strings

  21. Mapping Between Signed & Unsigned • Mappings between unsigned and two’s complement numbers:keep bit representations and reinterpret Unsigned Two’s Complement T2U x ux T2B B2U X Maintain Same Bit Pattern Two’s Complement Unsigned U2T ux x U2B B2T X Maintain Same Bit Pattern

  22. T2U U2T Mapping Signed  Unsigned

  23. = +/- 16 Mapping Signed  Unsigned

  24. + - + + + + • • • • • • + + + + + + Relation between Signed & Unsigned Unsigned Two’s Complement T2U x ux T2B B2U X Maintain Same Bit Pattern w–1 0 ux x Large negative weight becomes Large positive weight

  25. Conversion Visualized • 2’s Comp.  Unsigned • Ordering Inversion • Negative  Big Positive UMax UMax – 1 TMax+ 1 Unsigned Range TMax TMax 2’s Complement Range 0 0 –1 –2 TMin

  26. Signed vs. Unsigned in C • Constants • By default are considered to be signed integers • Unsigned if have “U” as suffix 0U, 4294967259U • Casting • Explicit casting between signed & unsigned same as U2T and T2U inttx, ty; unsigned ux, uy; tx = (int) ux; uy = (unsigned) ty; • Implicit casting also occurs via assignments and procedure calls tx = ux; uy = ty;

  27. Casting Surprises • Expression Evaluation • If there is a mix of unsigned and signed in single expression, signed values implicitly cast to unsigned • Including comparison operations <, >, ==, <=, >= • Examples for W = 32: TMIN = -2,147,483,648 , TMAX = 2,147,483,647 • Constant1 Constant2 Relation Evaluation 0 0U -1 0 -1 0U 2147483647 -2147483647-1 2147483647U -2147483647-1 -1 -2 (unsigned)-1 -2 2147483647 2147483648U 2147483647 (int) 2147483648U 0 0U == unsigned -1 0 < signed -1 0U > unsigned 2147483647 -2147483648 > signed 2147483647U -2147483648 < unsigned -1 -2 > signed (unsigned) -1 -2 > unsigned 2147483647 2147483648U < unsigned 2147483647 (int) 2147483648U > signed

  28. SummaryCasting Signed ↔ Unsigned: Basic Rules • Bit pattern is maintained • But reinterpreted • Can have unexpected effects: adding or subtracting 2w • Expression containing signed and unsigned int • int is cast to unsigned!!

  29. Today: Bits, Bytes, and Integers • Representing information as bits • Bit-level manipulations • Integers • Representation: unsigned and signed • Conversion, casting • Expanding, truncating • Addition, negation, multiplication, shifting • Summary • Representations in memory, pointers, strings

  30. w X • • • • • • X • • • • • • w k Sign Extension • Task: • Given w-bit signed integer x • Convert it to w+k-bit integer with same value • Rule: • Make k copies of sign bit: • X = xw–1 ,…, xw–1 , xw–1 , xw–2 ,…, x0 k copies of MSB

  31. Decimal Hex Binary 15213 x 3B 6D 00111011 01101101 15213 ix 00 00 3B 6D 00000000 00000000 00111011 01101101 -15213 y C4 93 11000100 10010011 -15213 iy FF FF C4 93 11111111 11111111 11000100 10010011 Sign Extension Example short intx = 15213; int ix = (int) x; short inty = -15213; intiy = (int) y; • Converting from smaller to larger integer data type • C automatically performs sign extension

  32. Summary:Expanding, Truncating: Basic Rules • Expanding (e.g., short int to int) • Unsigned: zeros added • Signed: sign extension • Both yield expected result • Truncating (e.g., unsigned to unsigned short) • Unsigned/signed: bits are truncated • Result reinterpreted • Unsigned: mod operation • Signed: similar to mod • For small numbers yields expected behavour

  33. Today: Bits, Bytes, and Integers • Representing information as bits • Bit-level manipulations • Integers • Representation: unsigned and signed • Conversion, casting • Expanding, truncating • Addition, negation, multiplication, shifting • Representations in memory, pointers, strings • Summary

  34. • • • • • • • • • • • • Unsigned Addition u • Standard Addition Function • Ignores carry output • Implements Modular Arithmetic s = UAddw(u , v) = u + v mod 2w Operands: w bits v + True Sum: w+1 bits u + v UAddw(u , v) Discard Carry: w bits

  35. Visualizing (Mathematical) Integer Addition • Integer Addition • 4-bit integers u, v • Compute true sum Add4(u , v) • Values increase linearly with u and v • Forms planar surface Add4(u , v) v u

  36. 2w+1 2w 0 Visualizing Unsigned Addition • Wraps Around • If true sum ≥ 2w • At most once Overflow UAdd4(u , v) True Sum Overflow v Modular Sum u

  37. • • • • • • • • • • • • Two’s Complement Addition u • TAdd and UAdd have Identical Bit-Level Behavior • Signed vs. unsigned addition in C: int s, t, u, v; s = (int) ((unsigned) u + (unsigned) v); t = u + v • Will gives == t Operands: w bits + v True Sum: w+1 bits u + v Discard Carry: w bits TAddw(u , v)

  38. TAdd Overflow • Functionality • True sum requires w+1 bits • Drop off MSB • Treat remaining bits as 2’s comp. integer True Sum 0 111…1 2w–1 PosOver TAdd Result 0 100…0 2w –1 011…1 0 000…0 0 000…0 1 011…1 –2w –1–1 100…0 NegOver 1 000…0 –2w

  39. Visualizing 2’s Complement Addition • Values • 4-bit two’s comp. • Range from -8 to +7 • Wraps Around • If sum  2w–1 • Becomes negative • At most once • If sum < –2w–1 • Becomes positive • At most once NegOver TAdd4(u , v) v u PosOver

  40. Multiplication • Goal: Computing Product of w-bit numbers x, y • Either signed or unsigned • But, exact results can be bigger than wbits • Unsigned: up to 2w bits • Result range: 0 ≤ x * y ≤ (2w – 1) 2 = 22w – 2w+1 + 1 • Two’s complement min (negative): Up to 2w-1 bits • Result range: x * y ≥ (–2w–1)*(2w–1–1) = –22w–2 + 2w–1 • Two’s complement max (positive): Up to 2w bits, but only for (TMinw)2 • Result range: x * y ≤ (–2w–1) 2 = 22w–2 • So, maintaining exact results… • would need to keep expanding word size with each product computed • is done in software, if needed • e.g., by “arbitrary precision” arithmetic packages

  41. • • • • • • • • • • • • • • • Unsigned Multiplication in C • Standard Multiplication Function • Ignores high order w bits • Implements Modular Arithmetic UMultw(u , v) = u · v mod 2w u Operands: w bits * v u · v True Product: 2*w bits UMultw(u , v) Discard w bits: w bits

  42. • • • • • • • • • • • • • • • Signed Multiplication in C • Standard Multiplication Function • Ignores high order w bits • Some of which are different for signed vs. unsigned multiplication • Lower bits are the same u Operands: w bits * v u · v True Product: 2*w bits TMultw(u , v) Discard w bits: w bits

  43. • • • Power-of-2 Multiply with Shift • Operation • u << kgives u * 2k • Both signed and unsigned • Examples • u << 3 == u * 8 • u << 5 - u << 3 == u * 24 • Most machines shift and add faster than multiply • Compiler generates this code automatically k u • • • Operands: w bits * 2k 0 ••• 0 1 0 ••• 0 0 u · 2k True Product: w+k bits 0 ••• 0 0 Discard k bits: w bits UMultw(u , 2k) ••• 0 ••• 0 0 TMultw(u , 2k)

  44. ••• ••• Unsigned Power-of-2 Divide with Shift • Quotient of Unsigned by Power of 2 • u >> kgives  u / 2k  • Uses logical shift k u Binary Point ••• Operands: / 2k 0 ••• 0 1 0 ••• 0 0 Division: u / 2k . 0 0 ••• 0 0 ••• Result: u / 2k  0 0 ••• 0 0 •••

  45. ••• ••• Signed Power-of-2 Divide with Shift • Quotient of Signed by Power of 2 • x >> kgives  x / 2k  • Uses arithmetic shift • Rounds wrong direction when u < 0 k x Binary Point ••• Operands: / 2k 0 ••• 0 1 0 ••• 0 0 Division: x / 2k . 0 ••• ••• RoundDown(x/ 2k) Result: 0 ••• •••

  46. Correct Power-of-2 Divide • Quotient of Negative Number by Power of 2 • Want  x / 2k  (Round Toward 0) • Compute as  (x+2k-1)/ 2k  • In C: (x + (1<<k)-1) >> k • Biases dividend toward 0 Case 1: No rounding k Dividend: u 1 ••• 0 ••• 0 0 +2k –1 0 ••• 0 0 1 ••• 1 1 Binary Point 1 ••• 1 ••• 1 1 Divisor: / 2k 0 ••• 0 1 0 ••• 0 0  u / 2k  . 0 1 ••• 1 1 1 ••• 1 ••• 1 1 • Biasing has no effect

  47. Correct Power-of-2 Divide (Cont.) Case 2: Rounding k Dividend: x 1 ••• ••• +2k –1 0 ••• 0 0 1 ••• 1 1 1 ••• ••• Incremented by 1 Binary Point Divisor: / 2k 0 ••• 0 1 0 ••• 0 0  x / 2k  . 0 1 ••• 1 1 1 ••• ••• Incremented by 1 • Biasing adds 1 to final result

  48. Today: Bits, Bytes, and Integers • Representing information as bits • Bit-level manipulations • Integers • Representation: unsigned and signed • Conversion, casting • Expanding, truncating • Addition, negation, multiplication, shifting • Summary • Representations in memory, pointers, strings

  49. Arithmetic: Basic Rules • Addition: • Unsigned/signed: Normal addition followed by truncate,same operation on bit level • Unsigned: addition mod 2w • Mathematical addition + possible subtraction of 2w • Signed: modified addition mod 2w (result in proper range) • Mathematical addition + possible addition or subtraction of 2w • Multiplication: • Unsigned/signed: Normal multiplication followed by truncate, same operation on bit level • Unsigned: multiplication mod 2w • Signed: modified multiplication mod 2w (result in proper range)

  50. Why Should I Use Unsigned? • Don’t Use Just Because Number Nonnegative • Easy to make mistakes unsigned i; for (i = cnt-2; i >= 0; i--) a[i] += a[i+1]; • Can be very subtle #define DELTA sizeof(int) inti; for (i = CNT; i-DELTA >= 0; i-= DELTA) . . . • Do Use When Performing Modular Arithmetic • Multiprecision arithmetic • Do Use When Using Bits to Represent Sets • Logical right shift, no sign extension

More Related